The present invention relates to a process for applying a coating to honeycomb structures in an exhaust gas converter which contains two such honeycomb structures in a common housing or enveloping tube, which are arranged one after the other and are separated from each other by a gap. The coating may be a catalytically active coating, an absorbing or adsorbing coating or a combination of these types of coating in an exhaust gas converter, which is deposited onto the honeycomb structure by using a coating dispersion of finely divided solids and optionally dissolved compounds.
More particularly, the present invention enables the coating of a plurality of separate honeycomb structures after being installed in a common housing thereby eliminating the need to apply dispersion coatings to honeycombs prior to their being positioned in the catalytic converter housing.
The vast majority of catalysts for purifying exhaust gases from, for example, stationary and non-stationary internal combustion engines, in order to convert them to harmless components are produced by first coating inert solid monolithic carriers with a catalytically active coating. After coating with the catalysts, the carriers are inserted into a housing or converter canister which has inlet and outlet openings for the ingress and egress of exhaust gases. Together with the housing, the catalysts form so-called exhaust gas converters. In the simplest case, the housing may consist of a single enveloping tube.
Solid inert carriers in this art, generally termed "monoliths", may comprise honeycomb structures made from a wide variety of ceramic materials the most common of which are cordierite and mullite or from heat-resistant metallic alloys with the main constituents chromium, aluminum and iron, or chromium, aluminum, iron and nickel. The honeycomb monolith has a cellular structure with parallel flow channels through which the exhaust gas can flow without a substantial loss of pressure due to the exhaust gas converter.
Ceramic honeycomb structures are generally obtained by the continuous extrusion of ceramic materials followed by drying and calcination. The cellular structure (square, triangular, etc.), cell wall thickness and cell density in the honeycomb structure can be freely selected according to each purpose and convenience as is known in the art. As a result of manufacturing tolerances (e.g. deformation by bending), however, this type of ceramic honeycomb structure is only available in restricted lengths.
Metallic honeycomb structures generally consist of a comparatively thin, shaped strip of metal foil of restricted width which stretches continuously from one end of the honeycomb structure to the other, which is housed in a tubular jacket mostly projecting to the outside and connected to this using an appropriate joining procedure. The metallic honeycomb structure itself is produced by methods of fabrication such as rolling up, folding or stacking one or more strips of metal foil, optionally interlaced with at least one length of sheet metal. The strip of metal foil may consist of smooth and/or corrugated, or similarly shaped foil, which for its part may have punched slots or openings of any type so that a shaped structure is formed which is permeable to gas in at least the direction of flow of the exhaust gas. Corresponding metallic honeycomb structures are described in, for example, EP-A 0 220 468, EP-A 0 245 736, DE-A 40 24 942, EP-A 0 484 364 and WO 89/10470.
Since the honeycomb structure and thus the final catalysts are only available in restricted lengths, two separate catalysts are frequently arranged one after the other in an exhaust gas converter housing. Other reasons for arranging catalysts in a sequence in the housing are, for instance, the use of carriers with different cell densities to minimize the pressure loss or to improve mixing of the exhaust gas after partial conversion of the pollutants in the first catalyst. Improved exhaust gas purification values may be produced by fresh mixing of the exhaust gas before it enters the second catalyst.
To reduce hydrocarbon emissions during a cold start, two honeycombs may also be combined in one exhaust gas converter housing, one of which is provided with a hydrocarbon adsorbing coating and the second being provided with a catalytically active coating.
In the case of ceramic catalysts, individual honeycomb structures, mostly with the same cross-section, are inserted at a specific distance from each other in the exhaust gas converter housing which is then sealed at the sides by welding. In the case of metallic catalysts with at least one enveloping tube which projects beyond the honeycomb structure on one side, the enveloping tubes for two honeycomb structures may also be directly welded to each other end-to-end to build up the exhaust gas converter housing. Mixed systems made from ceramic and metallic catalysts are also known.
In the case of known exhaust gas converters, the catalysts are produced by coating the inert honeycomb structures with a catalytically active coating before building them into the converter housing. To do this, the honeycomb structures are typically immersed in an aqueous coating dispersion of metal oxides with a high surface area such as, for example, .gamma.-aluminum oxide and cerium oxide optionally with other additives as promoters in the form of solids or dissolved compounds. After immersion, the flow channels in the honeycomb structure are freed of excess coating dispersion by blowing through with compressed air, dried and calcined. Such dispersion and immersion techniques are widely used in the art. They are used to form coatings on carriers and are generally referred to as "wash coats".
The catalytically active components, mostly noble metals in the platinum group and/or promoters, may be added to the coating dispersion before coating the honeycomb structure or incorporated into the coating after calcining the wash coat, by impregnating the honeycomb with aqueous solutions of soluble compounds of these noble metal and promoter components.
As an alternative to immersion, the honeycomb structure may also be coated with the coating dispersion by pumping it in, absorbing it or pouring it through the structure.
Thus, typical exhaust gas catalysts are a composite structure of inert carrier, coated with a wash coat and a noble metal.
For manufacturing simplicity and for cost-saving purposes, it is desirable, especially in the case of metal catalysts and the increasingly being used, externally heatable, catalysts in series with catalysts, to coat the two honeycomb structures in an exhaust gas converter consisting of two honeycomb structures only after incorporation in the converter housing. However, simply transferring the coating process for separate honeycomb structures to the finally produced and assembled exhaust gas converter is not really satisfactory because the turbulent current of air which is formed between the two honeycomb structures during the blowing out procedure can only inadequately remove the coating dispersion from the internal end faces of the two honeycomb structures and thus channel narrowing or even channel blockages can occur.
Thus, an object of the present invention is to improve coating processes for two honeycomb structures incorporated into one exhaust gas converter whereby the honeycomb structure can be coated to achieve the same quality results as would have been possible by separately coating the honeycomb structures before incorporating them into the converter housing.